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Old 16th October 2012, 03:32 AM   #1
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Default SMD redesign or back to through hole?

Doing some research and found your forum. Like the input I see so here goes;

Designed an audio system I call a Dual Transceiver Interface or DTI which mixes the audio output of two FRS/GMRS transceivers. It enables a user to monitor both radios and select either to transmit on from a single speaker mic.

The prototype works great however I made the huge mistake of employing 4.7 uF ceramic SMD caps for coupling on the inputs. (On the breadboard I used polarized electrolytic's. It's what I had on hand). I've just added another stage to up the output for better sound when using the speaker as opposed to the usual plug in ear piece unfortunately the audio quality degraded considerably. I've cleaned this up by going to 47 uF/25 V coupling caps on the inputs and between stages and 220 uF on the output. Again on the bread board this is done with aluminum electrolytic capacitors.

Here's the concern - I can put SMD electrolytic caps in the design but things start getting big again. What SMD (0805 to 1210 range) caps do you guys use that are good for audio that would recreate or improve the bread board sound (terrific bass) I'm getting? Can someone recommend a good material? Most film styles don't seem to come in this range.
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Old 16th October 2012, 04:27 AM   #2
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Do you understand the basics of the high-pass filter you're creating with the coupling caps you're using?

If not, would you mind posting a schematic so we can see?
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Old 16th October 2012, 05:11 AM   #3
jitter is offline jitter  Netherlands
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Some SMD components actually take up more space on a pcb than their through hole counterparts.
Radial electrolytic caps are a good example of that, and I'm guessing it's what you're thinking of using. The trough hole part has the leads tucked under the housing whereas the SMD part has them sticking out at the sides. As a result, the footprint of the latter might need more space than the former.
When space is limited, a careful selection of components must be considered.
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Old 17th October 2012, 07:16 PM   #4
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Thanks for your interest. I often combine smd and leaded components in my designs and am very familiar with the various footprints. My usual stuff involves logic or motor control circuits on double sided boards. This is my first foray into audio. This particular circuit is a simple mixer and amp built around 741 and 386 op amps. The only thing fancy about it are the digital pots used for volume controls.

The circuit works great however I used ceramic smd caps because I had lots. I've refined the sound and boosted the gain to what I want but it's on a bread board with leaded alum electrolytic capacitors in the design. What I'm looking for is advice on selecting good quality smds for the pcb. Whats considered the standard in audio? Thin film? Polyester? This isn't a studio quality system. It's an audio mixer and transmitter selector that will be used harsh conditions. It's gonna take a pounding actually.
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Old 17th October 2012, 08:13 PM   #5
rsavas is offline rsavas  Canada
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I have used 10uF/50V/X7R in a 1210 package and they sound okay to me!!
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Old 17th October 2012, 09:33 PM   #6
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If you use ceramic capacitors please Be aware that some materials tend to have lower capacity when charged to a voltage close to their rating. So they have a lower value than expected with a high DC voltage bias across them and distortion if there is large signal across them. Second problem may show in filters. X7R is better than other materials.
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Old 17th October 2012, 10:56 PM   #7
rsavas is offline rsavas  Canada
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Thanks for the info on MLC cap's, missing info on a data sheet probably. I use them for decoupling, for which they are excellent and for coupling, well you have read the papers about them, not the best there is, but it really depends on the system as a whole. Use them for signal coupling in a Class "D" amp, not a problem for me, very low bias voltage. For filters, probably not the best, due to accuracy, NPO(COG) another story.

Rick
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Old 18th October 2012, 03:04 AM   #8
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Thanks very much for the input. I'll take a look for X7R caps. I have plans to build myself an audio system this winter so I'll be checking back here for sure. It's great getting nice sounds out of a new circuit instead of clicks, beeps, flashing lights...or less lol.

I noticed some interesting changes to the sound while experimenting with this little circuit. Looking forward to trying my hand at his stuff.

Great forum.
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Old 18th October 2012, 04:13 AM   #9
qusp is offline qusp  Australia
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if you can still get them, Wima PPS SMD film caps
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Old 18th October 2012, 04:20 AM   #10
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by Copper Dog View Post
This is my first foray into audio. This particular circuit is a simple mixer and amp built around 741 and 386 op amps.
For decent sount quality and low price, I'd use NE5532 instead of LM741.
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